"I sent a photo of the droppings to my brother, he said mouse. The neighbour said rat. I bought mouse snap traps and they never went off. Three weeks later we found a dead brown rat behind the boiler." This homeowner in Créteil lost nearly a month to a misidentification. His traps were too small, badly placed, and baited with cheese — a cliché that matches neither species.
Identification is the first step in any rodent control effort, and it is the one most often skipped. Yet everything follows from it: trap size, placement, bait, the diameter of the holes to seal, the height to inspect, and even the question of whether the animal is legally protected or not.
This blog has already covered mechanical mouse trapping, sealing materials and high-level access points. This article tackles a separate subject, one that comes before all the rest: reading the clues — droppings, marks, noises, smells, gnaw damage — to find out who is moving around your home.
Droppings: the first clue, and the most reliable
A rodent on the move leaves droppings constantly, because it defecates as it travels. That is why droppings are the most usable clue: there are plenty of them, they map the routes used, and their shape is characteristic of the species.
| Species | Dropping size | Shape | Where you find them |
|---|---|---|---|
| House mouse (Mus musculus) | 3 to 7 mm | Slim spindle, pointed at both ends, shiny black when fresh | Drawers, back of cupboards, behind appliances, along skirting boards |
| Brown rat (Rattus norvegicus) | 15 to 20 mm | Thick grain of rice, rounded end, in piles | Cellar, crawl space, around inspection chambers, bin store, garden shed |
| Black rat (Rattus rattus) | 10 to 15 mm | Slimmer and more curved than the brown rat's, pointed | Loft, on top of beams, ridge line, insulation |
| Wood mouse (Apodemus sylvaticus) | 4 to 6 mm | Similar to the house mouse's, often paler, less smelly | Garage, garden shed, outbuilding, seed store |
| Edible or garden dormouse | 8 to 12 mm | Elongated, often grouped into a latrine | Loft, roller-shutter housing, glass wool |
| Bat | 3 to 8 mm | Crumbly, crush into a shiny insect powder | Under a beam, always directly below an exit point |
Two practical benchmarks that avoid 80% of mistakes:
- The grain rule. A mouse dropping looks like a very slim black grain of rice. A brown rat dropping looks like an olive stone. There is nothing in between: if you are torn between the two, it usually means both species are present, or that you are looking at young rat droppings.
- The crush test. A fresh rodent dropping is soft and shiny; after a few days it hardens, dulls and turns grey. A "dropping" that crumbles into glittering powder does not come from a rodent but from a bat — a fully protected species in France (arrêté du 23 avril 2007, the order of 23 April 2007), which must not be displaced or treated. The case is covered in detail in our article on noises in the loft.
A hygiene point: never handle rodent droppings with bare hands and never sweep them up dry. The INRS (French National Research and Safety Institute) points out that sweeping or vacuuming puts contaminated dust back into the air. Spray a disinfectant detergent, let it work, pick up with paper towel, and wear disposable nitrile gloves and an FFP2 mask before cleaning a cellar floor or soiled insulation. Leptospira bacteria, responsible for leptospirosis, survive several weeks in damp conditions according to Santé publique France (the French public health agency).
How many droppings, for how many animals?
A mouse produces 50 to 80 droppings a day, a brown rat 30 to 40. A rough count therefore tells you something about the scale of the problem:
- a few droppings in a single drawer → one animal, a recent visit, situation still simple;
- dozens of droppings along several distinct routes, in two different rooms → an established population, breeding likely;
- latrines (accumulations in one spot, several centimetres deep) → long-standing presence, often several months, typical of lofts and shutter housings.
A professional's tip: clean a zone completely, note the date, come back 48 hours later. Whatever reappears is necessarily active. That is more informative than any photo posted on a forum.
Grease marks, smear marks and runways
Rodents hug walls and keep using the same routes. Their coat, loaded with sebum and dust, gradually deposits a grey-black film on the surfaces it rubs against.
Look for these smear marks:
- in the angle between a wall and a cellar floor, 2 to 5 cm up;
- along the bottom edge of a garage door;
- around an entry hole, as a dark halo;
- on a pipe or cable used as a walkway.
A well-marked halo signals long, regular use: this is not a lost rodent, it is an established route. In brown rats these marks are clearly visible to the naked eye. In mice they are fainter and show up better with a torch held almost parallel to the surface, in grazing light. An adjustable-beam torch is frankly more useful here than a diffuse head torch, because it is the lighting angle that reveals the deposit.
For doubtful routes, the old talc method remains unbeatable: dust a thin layer of flour or talc over a 30 cm wide strip in front of a suspect hole, and read the prints the next day. You will see the difference immediately:
- prints 1 to 2 cm with a thin tail drag mark → mouse or wood mouse;
- prints 2 to 4 cm, widely spaced, thick tail drag mark → brown rat;
- no tail drag but broad pads and claws → stone marten or pine marten, a different subject with different regulations.
Gnaw damage: the most telling signature
A rodent's incisors grow continuously and must be worn down. What it gnaws, and how, gives away the species.
The mouse makes small, neat holes with nibbled edges, 2 to 3 cm across, in a skirting board, a cardboard box, a bag of pasta. It goes preferentially for soft packaging and the corners of boxes. It climbs effortlessly and weighs almost nothing: you will find damage in high kitchen cupboards.
The brown rat makes openings 5 to 8 cm across, with torn edges, in the bottom of a door, a chipboard panel, rigid plastic, a PVC waste pipe, sometimes an electrical conduit. Its bite marks leave two clearly visible parallel grooves. It can also burrow: a brown rat burrow along a house wall is 5 to 8 cm in diameter, with a small tongue of soil in front of it.
The edible and garden dormouse readily attack cables and conduits in lofts, and hollow out stored fruit. They leave walnut shells neatly opened into two halves — a brown rat, by contrast, shreds them.
The case of gnawed cables deserves particular attention: it is the main source of fire risk and costly breakdowns, whether in a loft or under a bonnet. We covered it for vehicles parked at home. A simple connected smoke alarm in a converted loft prevents nothing, but it warns early — and it costs less than rebuilding a roof structure.
